||The main topic of the study is the attestation of deverbative nouns ending in -м-, which are formed on a Slavonic basis without intermediary -ь- or -ъ-. The numerous dialect dictionaries and vocabularies of the East Slavonic languages and the most important dictionaries of all Slavonic languages form the main source of lexical material for the etymologization of the words in question. Seventy seven etymologies (most of them new) of East Slavonic deverbative nouns ending in *-мъ *-ма/*-мо are presented in the study. New etymologies are proposed, among others, for Literary Russian глум, дума, корм, корма, космы, кум(а), налим, пасмо, терем, угрюмый, хоромо, чума, шум and яма. On the basis of the proposed etymologies it is possible to give many new examples of the simplification of Common Slavonic word-internal consonant clusters.